Hazard Assessment of Large-Scale Releases of Combustible Chemicals
Greg Jackson and Arnaud TrouvéUniversity of Maryland, College Park
e-mail contact: [email protected]
Tom McGrath & Bill HinckleyNSWC – Indian Head Division
CECD Overview MeetingMay 14, 2007
• Safe dispersion
• Basic scenario:
Accidental release of gaseous fuelin ambient air
Turbulent mixing of fuel and air(delayed ignition)
Ignition(in flammable region of vapor cloud)
Combustion
• Explosion: detonation (blast)• Flash fire: deflagration (no blast)• Fireball: diffusion flame
• Formation of a large flammable cloud• Formation of a large ultra-rich cloudFuelAir Fuel
Fuel + Air
Air Fuel
Ultra-lean
Air Fuel
Flammable
Ultra-lean
Ultra-rich
Air Fuel
Ultra-lean
Ultra-rich
Air Fuel
Ignition
Fuel
PremixedFlame
Fuel
Flammable
Ultra-lean
Ultra-rich
Air Fuel
Ultra-lean
Ultra-rich
Air
Air Fuel
DiffusionFlame
Formation, Ignition, and Combustion of Fuel Vapor Clouds
Advanced Modeling Approach• Start to finish modeling of release, dispersion, and explosion or fire of
large scale chemical release requires multiple physical models – Spill modeling not yet implemented although explored– Dispersion requires convective/diffusive modeling– Detonation requires convective/reactive modeling with shock capturing
– Deflagration requires convective/diffusive/reactive models
• Approach to integrate/modify existing codes – Dispersion: Fire Dynamic Simulator (FDS) developed by NIST– Detonation: GEMINI by NSWC IHDIV and enhanced
• Enhanced by McGrath et al. to support gas-phase reactions/detonations– Deflagration: Fire Dynamic Simulator (FDS) by NIST
• Enhanced by Trouvé et al. for premixed and partially premixed combustion– Structural Response: In-house development of UMCP based on existing pressure-
impulse techniques
• Experiments in attempt to validate submodels
Detonation – Large Scale Model TestingPropane Vapor Release
• Pressure contours plotted at selected times during event
• Detonation wave travels through fuel/air dispersion and continues to propagate as a blast wave
• Blast reflects off target structure
• Detonation fails to consume all fuel present in domain
– Fuel mass fraction plotted at final simulation time
– Significant portions exist outside of detonability limits and do not react
– Remaining fuel may burn as a large fireball upon mixing with air
Dispersion – Simulation of Large Scale LNG Spill Vaporization
• Simulation of LNG spill
– 1.0 m/s crosswind over 10 m high ship
– 30 X 30 m LNG spill downstream of ship
– Constant heat transfer coefficient (155 W/m2*K)
• Fuel remains along surface but flammable regions rise well off surface
• Structure of dispersion shows buoyant plumes but hesitancy to interpret this as physically realistic
• Temperature remains cold in fuel rich regions
• Risk of large-scale fire possible but likely not flash fire due to strong gradients of fuel concentration.
Gas Temperature (°C)
pool location
pool location
CH4 mole fractions
~ flammability region
Dispersion – Flammable Mass as a Function of Wind Speed
0
50
100
150
200
250
300
0 100 200 300 400 500
Time fom initial vaporization (s)
Fla
mm
able
mas
s (k
g)
Uwind = 0.5 m/sUwind = 1.0 m/sUwind = 2.0 m/s
• Simulation of LNG spill vaporization downstream of a structure similar to a ship show that flammable mass created decreases with winds over the structure > 1 m/s.
• Results like this make a strong statement on hazard assessment.
Large Eddy Simulations for Formation, Ignition, and Transient Combustion of Fuel Vapor Clouds:
Arnaud Trouvé – University of Maryland
• Trouvé et al. have been developing sub-grid models that can couple turbulent premixed flame ignition and transition to partially premixed and fully non-premixed turbulent combustion.
• This capability is absolutely critical for large-scale fire hazard modeling where large chemical releases must be initially ignited as premixed mixtures and may likely transition over to a non-premixed steady fire scenario.
ign
cLu
jF
TcLu
jj
j
ccs
x
c
Sc
s
xuc
xc
t
~1~6
4
)~
)/616
(()~~(~
• Premixed combustion models added to FDS to capture deflagration propagation as well as cloud ignition processes
• Combustion model based on governing equations for the LES-filtered progress variable c:(Boger et al., Proc. Combust. Inst. 1998; Boger & Veynante, 2000)
• Variations of laminar flame speed with mixture strength are described using a presumed polynomial function of fuel properties including flammability limits
UnburntReactants
1~ c0~ c
Burnt Products
TurbulentFlame Front
Enhanced FDS Combustion Models – Premixed Flamesfor ignition and deflagration propagation
stZLFLZ
UFLZ
stLS ,
Flammable domain
cm 5.2)( 3/1 zyx
MJ 55kg 2.1
heptane :Fuel
F
F
Em
• Model problem: ignition of a fuel vapor cloud in a sealed compartment
– Uniform mesh (160×160×120) = 3,072,000
Coupled Combustion Models in Turbulent Flame Simulations
• Location and structure of premixed and non-premixed flames at t = 2.5 s
– Initiation of partially-premixed combustion
Premixed Non-premixed
Buoyant puff(vertical spread)
Expanding flame kernel (horizontal spread)
Coupled Combustion Models in Turbulent Flame Simulations
• Location and structure of premixed and non-premixed flames at t = 3 s
– Partially-premixed combustion
Premixed Non-premixed
Buoyant puff impinging on ceiling wall
Coupled Combustion Models in Turbulent Flame Simulations
• Time variations of global heat release rate of mixed premixed/diffusion flame
ignition
diffusion burningpartially-premixed
combustion
short time dynamics
Coupled Combustion Models in Turbulent Flame Simulations
• Continued efforts to seek further support– Meeting May 10, 2007 with ONI, DOE, FERC, and Coast Guard– Ongoing discussions (led by Bob Kavetsky) with DHS– Other efforts including collaborations with NIST
• Further improvements to deflagration modeling in FDS– Improved radiation modeling for coupling heat feedback to vaporization
(Jackson)– Testing of code to evaluate models ability to capture large-scale premixed to
diffusion flame transitions (Trouvé)• Further improvements to detonation modeling in Gemini
– Implementation of multi-phase detonations to evaluate high explosive threats as well as initial conditions of fuel vapor blasts (McGrath / Jackson)
• Efforts to establish dedicated computational facility at UMD for this project
Activities Going Forward